IRQ Settings and Mouse Installation

• Microsoft Bus Mouse, version 1.0

SUMMARY

The jumper on the bus mouse should be checked before the card is installed in any computer, especially if the card is moved from one machine to another. The board's jumper controls which interrupt request (IRQ) line is used. IRQ lines are used to facilitate information transfer from such I/O devices as disk controllers and serial ports. The jumper avoids IRQ conflicts with other devices already installed in the computer. Under each pair of pins on the card is a number between 2 and 5 (inclusive). The jumper selects the IRQ line. Microsoft ships the boards with the jumper set to IRQ 2 for installation in a typical IBM PC or PC XT. Because IBM changed the IRQ architecture in the PC AT by using IRQ 2 for the second IRQ controller, the bus mouse jumper is usually set to IRQ 5 on an AT.

This information applies to the Microsoft Bus Mouse for MS-DOS, OS/2, and Windows systems.

MORE INFORMATION

While these jumper settings are correct for most installations, you should verify which (if any) IRQ lines are being used by every device already installed in the machine. There can be only one active device per IRQ line. Refer to the technical manuals for each manufacturer's product or contact the manufacturer directly for this information. The following is an IRQ allocation table as defined in the IBM PC and AT technical reference manuals (other manufacturers' software, hardware and add-on boards must follow this convention in order to be IBM compatible):

                                        PC-AT
IRQ Line        PC, PC-XT       CTLR 1          CTLR 2

    0             Timer         Timer     | IRQ8  Clock
    1            Keyboard      Keyboard   | IRQ9  Redirected IRQ2
    2            Reserved       CTLR 2 <--| IRQ10 Reserved
    3              COM2          COM2     | IRQ11 Reserved
    4              COM1          COM1     | IRQ12 Reserved
    5           Hard disk        LPT2     | IRQ13 Coprocessor
    6          Floppy disk   Floppy disk  | IRQ14 Hard disk
    7             LPT1-3         LPT1     | IRQ15 Reserved

For example, a bus mouse is to be installed in an IBM PC-AT with an IBM PC-AT Serial/Parallel Adapter configured as COM1 and LPT1, respectively; a Color Graphics Adapter; and a multi-function card with 128K of memory and a serial port configured as COM2. On this computer, there is only one IRQ line still available on IRQ controller 1: IRQ 5. IRQ 2 is used by the AT's second IRQ controller and IRQ lines 4 and 3 are used by COM1 and COM2. The CGA does not use an IRQ line and the parallel port uses IRQ 7, which falls outside of the mouse's range. The bus mouse should be jumpered for IRQ 5, thereby using the last IRQ line in the normal IRQ range of 0 through 7 of the first IRQ controller. The user of this computer should be aware of this for future expansion.

The Enhanced Graphics Adapter (EGA) is now becoming a popular display card for PCs and XTs. The EGA includes a hardware feature that allows software to enable interrupts on IRQ2 to indicate the start of vertical retrace. Therefore, if an EGA and bus mouse are installed in a PC or XT, IRQ2 is no longer available for the mouse. In a full XT with a hard disk, two serial ports, an EGA, and a bus mouse, there is an IRQ line overlap between two devices. Therefore, one device must be sacrificed to free up an IRQ line for the bus mouse. This is not a design deficiency of the bus mouse; it is a fundamental design restriction in the PCs and XTs.

As discussed above, there are only eight IRQ lines in the PC and XT, of which four are used up by the motherboard and other standard equipment. The other four lines go quickly. The following types of expansion cards require that a free IRQ line be available:

• Network cards
• Bisync communication cards
• Tape back-up units
• Some clock/calendar hardware
• Serial communication cards
• EGAs
• Emulation boards
• Hard disk controllers

Once the hardware is installed, the software must be loaded. When the mouse driver loads, using either MOUSE.SYS from CONFIG.SYS or MOUSE.COM from a batch file or the keyboard, the file is loaded into memory and the driver then installs itself. This installation requires a few seconds. Various operations are undertaken, including mouse hardware initialization. This is the primary reason for the delay before the mouse installation message appears.

If the error message "MOUSE: Microsoft mouse not found!" appears, there can be a number of hardware-related causes, such as a broken mouse; however, the problem more likely is an IRQ contention problem either between the mouse and another device or between the serial ports.

Typical bus mouse related problems are between the mouse and the hard disk controller, that is, a bus mouse jumpered on IRQ 5 in an XT or IRQ 2 in an AT. The common symptom for this problem is the inability to perform a warm boot (CTRL+ALT+DEL). If the bus mouse is jumpered on the same line as a serial port, network card, or emulator card, irregular and unreproducible system crashes can occur. In Windows, with a bus mouse doubled up on the IRQ line used by a modem, the mouse support disappears when communications software is run.

The standard isolation procedure is to verify IRQ-line usage and if no problems are uncovered, the next step is to remove as much hardware as possible. In this way, the conflict should be uncovered between particular devices and a resolution of the problem follows.